The present invention relates to a pulse generator for electrical discharge machining, permitting to supply, at a high efficiency rate, short duration pulses to a machining zone between a workpiece an electrode tool, at a controllable voltage and frequency.
The present invention relates more particularly to a circuit for a pulse generator for electro-erosion machining providing electrical discharges between the electrode tool and the workpiece and which comprises a circuit including a capacitor connected across a DC power supply, the capacitor being charged from the DC power supply through an inductance connected in series, and at least one switching means for discharging the capacitor into a circuit comprising a pulse transformer, preferably of the saturable reactor type, connected to an electrical discharge circuit comprising the electrode tool and the workpiece.
Many circuits are known for providing current pulses by way of controlled discharge of a charged capacitor. In circuits where the capacitor is charged through a series inductance, it is well known that the maximum voltage across the capacitor may reach twice the voltage of the power supply, when the initial charge of the capacitor is equal to zero. Such an arrangement is disclosed in U.S. Pat. No. 2,835,785 which discloses several circuits connected in parallel for discharging a capacitor in a machining zone by means of a periodic control.
Another known circuit comprises in the discharge circuit of the capacitor a switching means, for example a thyristor, or a silicon controlled rectifier (SCR), which suppresses current inversion. In such cases, a fraction of the energy stored in the capacitor is not utilized in the discharge circuit and the voltage across the capacitor reaches a higher value at the end of each charge. U.S. Pat. Nos. 3,485,888 and 3,485,990 disclose such circuits.
However, although circuits of the type hereinbefore mentioned present advantages as far as permitting to obtain much higher discharge voltage than are obtained by more conventional circuits, they still present the disadvantage of supplying pulses having an energy level varying as a function of the conductivity of the discharge circuit. In addition, the energy level may also vary as a function of voltage variations across the charge circuit of the capacitor. The result is that the duration of voltage inversion across the thyristor terminals is not controllable.
It is known to regulate the voltage level of the electrical discharges by limiting or regulating the charge voltage of the capacitor to a predetermined value, for example by connecting a limiting or zener diode between the source and the capacitor. However, such an expedient is not applicable to the above mentioned circuits because it prevents the voltage across the inductance from reaching a zero value, and there results a progressive increase of the charge current and a progressive decrease of the time interval during which the voltage is inverted at the terminals of the thyristor. Stable operation of such a circuit cannot be obtained unless the average voltage of the charge circuit is lowered to a value which corresponds to the average voltage across the capacitor. By varying the ratio between the regulated voltage and the average voltage of the charge circuit the charge current of the capacitor can be controlled and, consequently, the duration during which the voltage is inverted across the thyristor terminals. Because this duration determines the switch-off speed of the thyistor, such an adjustment permits to increase the pulse frequency up to the limit of the dynamic characteristics of the thyristor with a high safety margin in the operation of the thyristor.
The present invention presents the advantage over the prior art circuits of utilizing an inductance in the capacitor charge circuit as an electrical energy accumulator such as to permit controlling the charge current during the increase of voltage across the capacitor, the capacitor being discharged when it reaches a predetermined constant voltage whose level may be adjusted to be either more or less than the power supply nominal voltage. Such results are accomplished by the circuit of the invention by providing means for adjusting the supply voltage of the charge circuit such as to maintain at a predetermined value a magnitude which is a function of the current flowing through the inductance, and by providing switching means which discharge the capacitor as soon as the voltage across the capacitor reaches a predetermined level.
The novel circuit of the invention presents the remarkable advantages of providing isoenergitic electrical discharges, or discharges of equal energy, with an adjustable voltage level which may be higher than the supply voltage. The maximum frequency of the pulses, which depends upon the dynamic characteristics of the switching means, may be safely reached. The pulse voltage level is unaffected by fluctuations of the supply voltage and the pulse frequency may be adjusted independently of the pulse voltage. In circuits where the means for adjusting the average voltage of the charge circuit consists of a second switching means closing and opening at a predetermined frequency, the circuit operates with a maximum energy efficiency. The invention therefore presents all the advantages of prior art circuits as far as the increase in pulse voltage level is concerned, while maintaining the pulse voltage level at a constant predetermined value in the course of machining.
The pulse generator of the present invention has particular advantages in EDM apparatus utilizing a wire electrode continuously fed to the machining zone, which is normally subject to relatively high wear. Under such conditions, it is possible to increase the machining efficiency by providing pulses of very short duration, at a high energy level and at high frequency. In addition, because of the presence of a pulse transformer between the discharge circuit and the machining circuit, the average voltage between the electrodes during the pulse cut-off intervals is practically nil, which avoids electrolysis of the machining fluid and which even permits to use water, as a machining fluid, with all the advantages resulting therefrom.